In a wireless communication system, a wireless communication terminal at a cell edge has a signal to interference and noise ratio (SINR) which is deteriorated and cannot acquire sufficient user rate by an influence by distance attenuation of desired wave power from an affiliated base station and an interference wave from a neighboring base station. As a technology that addresses the above problem and improves the user rate of the cell edge wireless communication terminal, a base station cooperation technology in which the base stations cooperate to transmit data to the wireless communication terminal was known. Even in long term evolution-advanced (LTE-A) (Non-patent Literature 3) which is a successor to 3rd generation partnership project (3GPP) long term evolution (LTE) (Non-patent Literature 1 and 2) which has been widely adopted globally as a 3.9th generation wireless communication system, and a first candidate of a 4th generation wireless communication system, it is expected that the base station cooperation technology will be included in a standard.
In the 3GPP, the base station cooperation technology is classified into two types of coordinated scheduling and joint processing and is under examination. The coordinated scheduling is a technology, that each base station makes a transmission signal have strong directivity by using beam forming (BF) and signals are prevented from being overlapped between neighboring base stations, to avoid interference and improve the SINR. Meanwhile, the joint processing is a technology in which the plurality of base stations transmits data to one terminal simultaneously to improve a transmission rate. A transmission mode at the time of simultaneously transmitting data from the plurality of base stations includes site diversity in which the plurality of base stations transmits the same signal and improves desired signal power in the terminal and multipoint MIMO in which MIMO transmission is performed by using a plurality of antennas of the plurality of base stations. Herein, an operation of the multipoint MIMO will be described with the operation being divided into single user (SU) transmission (one wireless communication terminal is selected and data is transmitted to the wireless communication terminal) and multi user (MU)-MIMO transmission (data is transmitted to a plurality of wireless communication terminals by applying MIMO). Further, in the following description, orthogonal frequency division multiple access (OFDM) adopted as downlink of LTE is assumed as a multiple access of a radio resource, but in the present description the multiple access of the radio resource is not limited to the OFDMA but may be applied to even other multiple access such as time division multiple access (TDMA), code division multiple access (CDMA), and the like.
In the SU transmission, the wireless communication terminal receives pilot signals from its affiliated base station and a neighboring base station, and estimates a channel. Further, the quality of the channel, the number of ranks of the MIMO, and a desired precoding matrix are calculated when the multipoint MIMO transmission is used based on the channel estimation result. The wireless communication terminal transmits at least one of them and a list of base stations that participate in cooperative transmission to its affiliated base station by using an uplink control signal. The base station receiving them notifies information thereon to a cooperation schedule unit serving to allocate the radio resource in base station cooperation. The cooperation schedule unit selects an optimal wireless communication terminal, a data transmission mode, used subcarriers, and the like based on the information thereon, and notifies the result to the cooperative transmission base station. Further, the optimal wireless communication terminal can be selected every subcarrier of the OFDMA. For example, when network MIMO transmission is performed between base stations 1 and 2 and wireless communication terminals 1 and 2, both base stations may perform multipoint MIMO communication with respect to the wireless communication terminal 1 in subcarriers 1 to 12 and both base stations may perform the multipoint MIMO communication with respect to the wireless communication terminal 2 in subcarriers 13 to 24.
As the multipoint MIMO transmission mode in the SU transmission, (1) open-loop MIMO transmission not requiring designation of the precoding matrix from the wireless communication terminal is performed and the wireless communication terminal performs reception in minimum mean square error (MMSE) or maximum likelihood detection (MLD) like general MIMO. (2) Closed-loop MIMO transmission such as Eigen space division multiplexing (E-SDM), and the like is used. (3) Transmission diversity using space time transmit diversity (STTD), and the like can be used. Even in any case, before transmission to the wireless communication terminal, base stations that perform cooperative transmission exchange required data and thereafter, each base station generates a signal based on each mode and transmits the generated signal to a target wireless communication terminal. The wireless communication terminal decodes the signal based on the mode selected by the base station, and receives data. Even in any mode of (1) to (3) described above, since the wireless communication terminal which is a target of the MIMO transmission receives its desired signal instead of interference from the base station that participates in cooperation, a channel capacity is significantly improved.
Meanwhile, even in the MU-MIMO transmission, the wireless communication terminal receives the pilot signal from its affiliated base station and the neighboring base station, and estimates the channel, similarly as one wireless communication terminal transmission. Further, the quality of the channel, the number of ranks of the MIMO, a desired precoding matrix, and a channel matrix with the plurality of base stations are calculated when the multipoint MIMO transmission is used based on the channel estimation result. The wireless communication terminal transmits at least one of them and a list of base stations that participate in cooperative transmission to its affiliated base station by using an uplink control signal. The base station receiving them notifies information thereon to a cooperation schedule unit serving to allocate the radio resource in base station cooperation. The cooperation schedule unit selects a combination of optimal wireless communication terminals, a data transmission mode, used subcarriers, and the like based on the information thereon, and notifies the result to the cooperative transmission base station. Further, the combination of the optimal wireless communication terminals can be selected every subcarrier of the OFDMA. For example, when network MIMO transmission is performed between base stations 1 and 2 and wireless communication terminals 1, 2, and 3, both base stations may perform network MIMO communication with respect to the wireless communication terminals 1 and 2 in subcarriers 1 to 12 and both base stations may perform the network MIMO communication with respect to the wireless communication terminal 2 and 3 in subcarriers 13 to 24.
As the multipoint MIMO transmission mode in the MU-MIMO transmission, (1) a zero forcing (ZF) mode in which a transmitter performs precoding by an inverse matrix of a channel matrix is used (Non-patent Literature 4). (2) A mode using dirty paper coding (DPC) of improving a channel capacity by using information on an interference signal is considered. The ZF can be implemented according to a simple principle, but amplification over an upper limit of transmission power is required according to the inverse matrix of the channel matrix, and as a result, there is a problem in that the channel capacity deteriorates. Meanwhile, the DPC is more excellent than the ZF in terms of channel capacity, but there is a problem in that a calculation amount is large. Accordingly, as one of the implementation modes of the DPC, a mode using LQ decomposition is known. In this mode, the channel matrix is resolved into the multiplication of a lower triangular matrix and a unitary matrix, and the transmitter performs preliminary equalizing processing based the lower triangular matrix and precoding by Hermitian transposition of the unitary matrix. This sequence is a calculation amount which can be installed in actuality, and further, since the unitary matrix is used in the precoding, extreme amplification of a signal amplitude as in the ZF does not occur. As a result, in the wireless communication terminal, interference from a neighboring cell is cancelled, and thus the channel capacity is improved.